The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Sickle cell disease (SCD) is caused by an abnormal type of hemoglobin called hemoglobin S (Hgb S). Hemoglobin is a protein inside red blood cells that carries oxygen. When oxygen is reduced inside the red blood cell, hemoglobin S molecules will polymerize forming long thin rods, elongating the red blood cell forming the sickle cell shape. The sickle-shaped cells deliver less oxygen to the body's tissues, are removed from the blood by the spleen, and can lodge in small capillaries that disrupt blood flow resulting in anemia and painful sickle crises.
Sickle cell disease is inherited as an autosomal recessive trait, which means the disease occurs in someone who has inherited hemoglobin S from both parents. Sickle cell disease is much more common in certain ethnic groups, affecting approximately two of every thousand Blacks born in the United States. The disease is most prevalent in Africa, particularly in the sub-Saharan area. It is also observed in Haiti, the Mediterranean area, as well as India. It is characterized by general weakness and pains in muscles and joints and is fatal, frequently at an early age. Untreated, victims usually die in early childhood but many who are treated can live into middle or even late adulthood. Someone who inherits hemoglobin S from one parent and normal hemoglobin A (Hgb A) from the other parent will have sickle cell trait (SCT). Someone who inherits hemoglobin S from one parent and another type of abnormal hemoglobin, like thalassemia, from the other parent will have another form of sickle cell disease, in which patients will present with characteristics of both sickle cell and thalassemia.
Thalassemia, like sickle cell disease, is also a disorder of hemoglobin. There are many forms of thalassemia but patients with certain forms can present to a physician similarly as a patient with sickle cell disease. Thalassemia is important to the invention of the present application for two reasons. First, as previously described, patients can inherit the sickle cell abnormality and a thalassemia abnormality causing their clinical presentation and their laboratory test results to appear similar to patients with sickle cell disease. And second, certain forms of thalassemia consistently produce imbalances in the globin chains that make up hemoglobin causing excess globin chains to aggregate and precipitate inside red blood cells.
Patients with sickle cell disease need continuous treatment, even when they are not having a painful crisis. For example, patients with sickle cell disease require supplementation with folic acid, an essential vitamin necessary for cell division, because of their rapid red blood cell turnover. Children are given prophylactic antibiotic therapy to prevent potentially life-threatening infections, which are the number one cause of death in this age group. Prevention of symptoms is also accomplished by administering a drug called hydroxyurea (Hydrea) or by administering blood transfusions to keep the normal hemoglobin level (Hgb A) high and the sickle hemoglobin level (Hgb S) low. Continuous treatment or therapy serves the purpose of managing and controlling symptoms, and limiting the frequency of crises.
During a sickle crisis, certain therapies may be necessary. Moreover, treatment of pain is critical. Painful episodes may be treated with analgesics and adequate liquid intake. Non-narcotic medications may be effective, but some patients will require narcotics. Hydroxyurea was found to help some patients by reducing the frequency of painful crises and episodes of acute chest syndrome, and by decreasing the need for blood transfusions. However, there has been some concern about hydroxyurea possibly causing leukemia, though there are no definitive data that hydroxyurea causes leukemia in sickle cell patients.
While bone marrow transplants can be curative, this therapy is prescribed in only a minority of patients, predominantly because of the difficulty in finding suitable donors and the high risk of the procedure (the drugs needed to make the transplant possible are highly toxic and the new bone marrow may attack the patient's tissues). Bone marrow transplants are also much more expensive than other treatments.
Attempts are being made to develop newer drugs, which include agents that work by trying to induce the body to produce more fetal hemoglobin or by increasing the binding of oxygen to sickle cells, both of which will decrease the amount of sickling. But as yet, hydroxyurea is the only widely used drug that is available for this form of treatment. Antibiotics and vaccines are typically given to prevent bacterial infections, which are common in children with sickle cell disease. Accordingly, early diagnosis of sickle cell disease in children is essential to providing early, life-saving treatment.
The classic hemoglobin solubility testing in which sickle cell patients are identified is based on solubility differences between the normal hemoglobins A, A2, and F and hemoglobin S. Hgb S results from the expression of a point mutation in the 6th position of the beta globin chain causing the amino acid glutamic acid to be replaced with valine. The classic hemoglobin solubility test uses a mild detergent, usually Saponin, to disrupt the red blood cell (RBC) membrane and release hemoglobin into solution. A reducing agent, usually sodium hydrosulfite, is present in order to reduce the hemoglobin to the deoxygenated form, thereby altering its quaternary conformation and resultant solubility characteristics.
Normal hemoglobins (Hgb A, A2, F) in the reduced state are soluble in 2.3 M phosphate buffer whereas Hgb S is insoluble. The insolubility of Hgb S produces a turbid solution that is interpreted as a positive test. Normal hemoglobins remain soluble producing a clear solution and a negative test. Unfortunately, both sickle cell heterozygotes (Hgb AS) and homozygotes (Hgb SS) produce a positive test, thereby requiring complex confirmatory testing, such as hemoglobin electrophoresis, HPLC or genetic testing, to distinguish zygosity.